FERMENTATION (INTRODUCTION) | Conceptional Explanations| Chemical Reactions

Despite the tremendous ATP yield obtained by oxidative phosphorylation, some chemoorganotrophic microbes do not respire because either they lack electron transport chains or they repress the synthesis of electron transport chain components under anoxic conditions, making anaerobic respiration impossible. Yet NADH produced by the Embden-Meyerhof pathway reactions during glycolysis (figure 9.5) must still be oxidized back to NAD. If NAD is not regenerated, the oxidation of glyceraldehyde 3-phosphate will cease and glycolysis will stop. Many microorganisms solve this problem by slowing or stopping pyruvate dehydrogenase activity and using pyruvate or one of its derivatives as an electron acceptor for the reoxidation of NADH in a fermentation process. There are many kinds of fermentations, and they often are characteristic of particular microbial groups. A few of the more common fermentations are introduced here, and several others are discussed at later points. Three unifying themes should be kept in mind when microbial fermentations are examined: (1) NADH is oxidized to NAD, (2) the electron acceptor is often either pyruvate or a pyruvate derivative, and (3) oxidative phosphorylation cannot operate, reducing the ATP yield per glucose significantly. In fermentation, the substrate is only partially oxidized, ATP is formed exclusively by substrate-level phosphorylation, and oxygen is not needed. Many fungi, protists, and some bacteria ferment sugars to ethanol and CO2 in a process called alcoholic fermentation. Pyruvate is decarboxylated to acetaldehyde, which is then reduced to ethanol by alcohol dehydrogenase with NADH as the electron donor. Lactic acid fermentation, the reduction of pyruvate to lactate is even more common. It is present in bacteria (lactic acid bacteria, Bacillus), protists (Chlorella and some water molds), and even in animal skeletal muscle. Lactic acid fermenters can be separated into two groups. Homolactic fermenters use the Embden-Meyerhof pathway and directly reduce almost all their pyruvate to lactate with the enzyme lactate dehydrogenase. Heterolactic fermenters form substantial amounts of products other than lactate; many produce lactate, ethanol, and CO2. LIKE, SHARE & SUBSCRIBE TO MY CHANNEL... THANKS